The present invention relates generally to moveable machines such as agricultural machines; and more particularly to a position-finding device for locating the position of a working edge for the machine, especially the edge of a crop, wherein a position-finding signal therefrom is supplied in the form of a steering adjustment value to a steering device on the machine.
A position-finding device for controlling a steering mechanism is known from DE 195 08 941 A1. In this device, a third sub position-finding device whose transmitter and receiver are directed at the worked field serves for producing a further reference value and normalization value. This arrangement however, leads to high levels of signal distortion and hence an unwanted, non-stable steering characteristic when the reflective properties of the worked field constantly vary e.g. due to differing amounts of straw lying on the stubble or if the field boundary lies in the reflection zone. The focus and alignment of the various sub position-finding devices is such that their reflection zones are to a large extent mutually independent and there is no interaction.
Furthermore, various position-finding devices for controlling a steering mechanism are known from DE 195 08 942 A1. One of these devices uses two tightly focussed, diverging light beams. At the position of maximum divergence, one of these beams falls on the worked field and the other on the unworked field and their reflections are used, when deviations occur, to provide an on-off type of control process in correspondence with their transit times. However, this results in a steering action that wavers undesirably along the intended path. Moreover, this document also discloses an ultrasonic position-finding device for controlling a steering mechanism using a single ultrasonic transmitter and receiver combination whose reflection zone overlaps each side of the working edge. This device utilizes the differing transit times of the echoes coming from the upstanding grain and the lower lying stubble. Should the crop alternately lie flat and then upright, or, if it is low and the alignment of the stalks or crops varies, then it is not possible to discriminate between the echoes from the crops and the echoes from the stubble so that the steering action becomes uncontrollable due to the lack of a suitable reference value.